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Abstract:

An imaging lens includes, in order from an object side, a positive first
lens group, a stop, and a positive second lens group. The first lens
group includes, in order from the object side, a first lens, which is a
negative meniscus lens having a convex object-side surface, and a second
lens having a positive power and including a convex image-side surface.
The second lens group includes, in order from the object side, a third
lens having a negative power and including a concave object-side surface,
a fourth lens having a positive power and including a convex image-side
surface, a fifth lens, which is a biconvex lens, and a sixth lens, which
is a meniscus lens having a negative power and including a convex surface
facing an image side. Each of the first to sixth lenses is a single
spherical glass lens.

Claims:

1. An imaging lens comprising:a first lens group having a positive power;a
stop; anda second lens group having a positive power,wherein the first
lens group, the stop, and the second lens group are arranged in this
order from an object side,the first lens group includes, in order from
the object side,a first lens, which is a meniscus lens having a negative
power and including a convex surface facing the object side, anda second
lens that has a positive power and includes a convex image-side
surface,the second lens group includes, in order from the object side,a
third lens that has a negative power and includes a concave object-side
surface,a fourth lens that has a positive power and includes a convex
image-side surface,a fifth lens, which is a biconvex lens, anda sixth
lens, which is a meniscus lens having a negative power and including a
convex surface facing an image side, andeach of the first to sixth lenses
is a single spherical glass lens.

2. The imaging lens according to claim 1,wherein the imaging lens
satisfies the following conditional expression:1.1<f36/f<1.7where f
indicates a focal length of an entire imaging lens system, and f36
indicates a focal length of the second lens group.

3. The imaging lens according to claim 1,wherein the imaging lens
satisfies the following conditional expression:-1.4>f1/f2<-0.7where
f1 indicates a focal length of the first lens and f2 indicates a focal
length of the second lens.

4. The imaging lens according to claim 1,wherein the imaging lens
satisfies the following conditional expression:1.1<R11/R12<2.0where
R11 indicates a curvature radius of an image-side surface of the fifth
lens, and R12 indicates a curvature radius of an object-side surface of
the sixth lens.

5. The imaging lens according to claim 1,wherein the imaging lens
satisfies the following conditional expression:νd3<20where νd3
indicates the Abbe number of the third lens at the d-line.

6. An imaging apparatus comprising:the imaging lens according to claim 1;
andan imaging device that converts an optical image formed by the imaging
lens into an electric signal.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001]This application is based upon and claims the benefit of priority
from the Japanese Patent Application No. 2008-276246 filed on Oct. 28,
2008; the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002]1. Field of the Invention

[0003]The present invention relates to an imaging lens that captures the
image of an object, and an imaging apparatus using the imaging lens.

[0004]2. Description of the Related Art

[0005]An in-vehicle imaging apparatus has been proposed which monitors the
front of a vehicle. The imaging apparatus is provided in the vehicle to
monitor the deviation of the vehicle from its lane or to monitor traffic
signs while the vehicle is traveling. Generally, an imaging lens having a
small size and a long back focal length has been used as an in-vehicle
imaging lens.

[0006]For example, a structure has been proposed in which a cemented lens
or a plastic aspheric lens is used as an imaging lens having a small size
and a long back focal length (see Japanese Patent Nos. 3254239, 3478643
(U.S. Pat. No. 5,682,269), 3723637, and 3964533).

[0007]Japanese Patent Nos. 3254239, 3478643(U.S. Pat. No. 5,682,269),
3723637, and 3964533 disclose imaging lenses using cemented lenses.
Japanese Patent No. 3254239 discloses an imaging lens that has a large
number of lenses including an aspheric lens having a relatively small
angle of view. Japanese Patent No. 3478643(U.S. Pat. No. 5,682,269)
discloses an imaging lens using a large number of meniscus lenses.
Japanese Patent No. 3723637 discloses an imaging lens having a total
length greater than the focal length. Japanese Patent No. 3964533
discloses an imaging lens that has a relatively high F number (Fno=4) and
a small angle of view and does not correspond to an increase in diameter.

[0008]However, the in-vehicle imaging lens, for example, is provided in
the vehicle in severe environments, such as a low-temperature environment
of 0° C. or less in a cold region and a high-temperature and
high-humidity environment in a tropical region (for example, a
temperature of 80° C. and a humidity of 80%). Therefore, a cement
of the cemented lens or the plastic lens is made of a material forming
the imaging lens is likely to be transformed or deformed, which may cause
deterioration of the optical performance of the imaging lens, for
example, a reduction in resolution. In addition, the optical performance
of the imaging lens may be lowered due to a variation in the shape or
refractive index of the imaging lens caused by a change in the
temperature of the imaging lens while capturing the image of an object.

[0009]Therefore, it is necessary to prevent the optical performance of the
imaging lens from being lowered due to the transformation or deformation
of each lens, or the variation in the refractive index of each lens when
the imaging lens is provided in a severe environment.

[0010]The problem of the deterioration of the optical performance of the
imaging lens is not limited to the in-vehicle imaging lens, but it also
arises in an imaging lens including a plastic lens or a cemented lens.

SUMMARY OF THE INVENTION

[0011]The invention has been made in order to solve the above-mentioned
problems, and an object of the invention is to provide an imaging lens
having a small size, a long back focal length, and high environmental
resistance and an imaging apparatus using the imaging lens.

[0012]According to an aspect of the invention, an imaging lens includes a
first lens group having a positive power, a stop, and a second lens group
having a positive power. The first lens group, the stop, and the second
lens group are arranged in this order from an object side. The first lens
group includes a first lens and a second lens. The first lens is a
meniscus lens having a negative power and including a convex surface
facing the object side. The second lens has a positive power and includes
a convex image-side surface. The second lens group includes, in order
from the object side, a third lens, a fourth lens, a fifth lens and a
sixth lens. The third lens has a negative power and includes a concave
object-side surface. The fourth lens has a positive power and includes a
convex image-side surface. The fifth lens is a biconvex lens.

[0013]The sixth lens is a meniscus lens having a negative power and
including a convex surface facing an image side. Each of the first to
sixth lenses is a single spherical glass lens.

[0024](where νd3 indicates the Abbe number of the third lens at the
d-line).

[0025]According to another aspect of the invention, an imaging apparatus
includes the imaging lens according to the above-mentioned aspect and an
imaging device that converts an optical image formed by the imaging lens
into an electric signal.

[0026]The single spherical glass lens means a single lens that is made of
only a glass material and includes a spherical object-side surface and a
spherical image-side surface.

[0027]According to the imaging lens and the imaging apparatus using the
image lens of the above-mentioned aspects, the imaging lens includes a
first lens group having a positive power, an aperture diaphragm, and a
second lens group having a positive power arranged in this order from an
object side. The first lens group includes a first lens, which is a
meniscus lens that has a negative power and includes a convex surface
facing the object side, and a second lens that has a positive power and
includes a convex image-side surface arranged in this order from the
object side. The second lens group includes a third lens that has a
negative power and includes a concave object-side surface, a fourth lens
that has a positive power and includes a convex image-side surface, a
fifth lens, which is a biconvex lens, and a sixth lens, which is a
meniscus lens that has a negative power and includes a convex surface
facing an image side, arranged in this order from the object side. Each
of the first to sixth lenses of the imaging lens is a single spherical
glass lens. Therefore, it is possible to obtain an imaging lens having a
small size, a long back focal length and high environmental resistance.

[0028]That is, both the lens closest to the object side in the first lens
group and the lens closest to the object side in the second lens group
have negative powers. Therefore, it is possible to increase the back
focal length to be greater than the length of the entire imaging lens
system.

[0029]The back focal length is an air equivalent distance from the lens
surface closest to the image side among the lens surfaces of the imaging
lens to the imaging surface of the imaging lens.

[0030]The lens closest to the object side in the entire imaging lens
system is a meniscus lens that has a negative power and includes a convex
surface facing the object side, and the lens closest to the image side in
the entire imaging lens system is a meniscus lens that has a negative
power and includes a convex surface facing the image side. Therefore, it
is possible to reduce the size of the imaging lens and prevent the
occurrence of field curvature or distortion.

[0031]The cement of the cemented lens or the plastic lens is made of a
material, such as a polymer material, having a melting point lower than
that of a glass material. The low-melting-point material is more likely
to be deformed by the influence of the temperature or humidity than the
glass material, and the optical characteristics thereof, such as the
refractive index, are also more likely to be changed. In addition, the
low-melting-point material is more likely to be transformed than the
glass material. Therefore, when the imaging lens is used for a long time,
the optical performance of the imaging lens is lowered.

[0032]According to the above-mentioned aspects of the invention, the
imaging lens is not made of the low-melting-point material, but is made
of only a glass member having high environmental resistance. Therefore,
it is possible to obtain an imaging lens and an imaging apparatus having
high environmental resistance. In addition, since the imaging lens
includes only the spherical lenses, it is possible to easily manufacture
the imaging lens, as compared to the imaging lens including the aspheric
lenses. As a result, it is possible to reduce apparatus costs.

[0033]When the environmental resistance of the imaging lens is increased,
the deterioration of the optical performance of the imaging lens caused
by the influence of an environment is reduced. For example, when the
environmental resistance of the imaging lens is high, the transformation
or deformation of the lens member is reduced and the deterioration of the
optical performance of the imaging lens is reduced even when the imaging
lens is used for a long time or it is provided in a low-temperature
environment or a high-temperature and high-humidity environment in a
short period of time. In addition, when the environmental resistance of
the imaging lens is high, the deterioration of the optical performance of
the imaging lens is reduced even when the temperature of the imaging lens
varies greatly from room temperature during image capture in a cold
region or a tropical region.

BRIEF DESCRIPTION OF THE DRAWINGS

[0034]FIG. 1 is a diagram schematically illustrating the structure of an
imaging lens according to an embodiment of the invention;

[0035]FIG. 2 is a cross-sectional view schematically illustrating the
structure of an imaging lens according to Example 1;

[0036]FIG. 3 is a cross-sectional view schematically illustrating the
structure of an imaging lens according to Example 2;

[0037]FIG. 4 is a cross-sectional view schematically illustrating the
structure of an imaging lens according to Example 3;

[0038]FIG. 5 is a cross-sectional view schematically illustrating the
structure of an imaging lens according to Example 4;

[0039]FIG. 6 is a cross-sectional view schematically illustrating the
structure of an imaging lens according to Example 5;

[0040]FIG. 7 is a cross-sectional view schematically illustrating the
structure of an imaging lens according to Example 6;

[0041]FIG. 8 is a cross-sectional view schematically illustrating the
structure of an imaging lens according to Example 7;

[0049]FIG. 16 is a diagram illustrating a vehicle provided with an
in-vehicle camera, which is an imaging apparatus using the imaging lens
according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0050]Hereinafter, an imaging lens and an imaging apparatus using the
imaging lens according to exemplary embodiments of the invention will be
described in detail with reference to the accompanying drawings.

[0051]FIG. 1 is a cross-sectional view schematically illustrating the
structure of an imaging apparatus using an imaging lens according to an
embodiment of the invention.

[0052]An imaging lens 20 shown in FIG. 1 is mainly used for an in-vehicle
imaging apparatus that captures a situation on the front side of a
vehicle, and focuses the image of an object on a light receiving surface
Jk of an imaging device 10, such as a CCD or a CMOS. The imaging device
10 converts an optical image formed by the imaging lens 20 into an
electric signal to obtain an image signal indicating the optical image.

[0053]The imaging lens 20 has an F number of 2.0 to 4.0 and a total angle
of view of about 60°. The imaging lens 20 has a small size and a
long back focal length and effectively corrects various aberrations.

<Basic Structure, Operation, and Effect of Imaging Lens>

[0054]First, the basic structure of the imaging lens 20 will be described.

[0055]The imaging lens 20 includes a first lens group U1 having a positive
power, an aperture diaphragm, and a second lens group U2 having a
positive power arranged in this order from an object side (the side of an
arrow -Z in FIG. 1) along an optical axis Z1.

[0056]The first lens group U1 includes a first lens L1, which is a
meniscus lens having a negative power and including a convex surface
facing the object side, and a second lens L2 that has a positive power
and includes a convex surface R4 on the image side (the side of an arrow
+Z in FIG. 1). The first and second lenses L1 and L2 are arranged in this
order from the object side.

[0057]The second lens group U2 includes a third lens L3 that has a
negative power and includes a concave object-side surface R6, a fourth
lens L4 that has a positive power and includes a convex image-side
surface R9, a fifth lens L5, which is a biconvex lens, and a sixth lens
L6, which is a meniscus lens having a negative power and including a
convex surface facing the image side. The third to sixth lenses are
arranged in this order from the object side.

[0058]Each of the first to sixth lenses L1 to L6 is a single spherical
lens made of a glass material.

[0059]That is, each of the lenses of the imaging lens 20 is a single
spherical glass lens, but is not a cemented lens.

[0060]A light receiving surface Jk of the imaging device 10 is provided as
an imaging surface R14 on which the image of an object is focused by the
imaging lens 20.

[0062]That is, R1 and R2 indicate the object-side surface and the
image-side surface of the first lens L1, respectively, and R3 and R4
indicate the object-side surface and the image-side surface of the second
lens L2, respectively. R5 indicates an aperture of the aperture diaphragm
St, and R6 and R7 indicate the object-side surface and the image-side
surface of the third lens L3, respectively. R8 and R9 indicate the
object-side surface and the image-side surface of the fourth lens L4,
respectively, and R10 and R11 indicate the object-side surface and the
image-side surface of the fifth lens L5, respectively. R12 and R13
indicate the object-side surface and the image-side surface of the sixth
lens L6, respectively, and R14 indicates the imaging surface of the
imaging lens 20, which is the light receiving surface Jk of the imaging
device 10, as described above.

[0063]According to the basic structure of the imaging lens 20, it is
possible to obtain an imaging lens having a small size, a long back focal
length, and high environmental resistance.

[0064]The back focal length (which is represented by Bf in the drawings)
is an air equivalent distance from the image-side surface R13 of the
sixth lens L6 to the imaging surface R14.

<Structure Further Limiting Basic Structure of Imaging Lens and
Operation and Effect Thereof>

[0065]Next, components further limiting the basic structure of the imaging
lens 20 and the operation and effect thereof will be described. The
components further limiting the basic structure of the imaging lens are
not necessarily required.

<<Structure for Limiting the Basic Structure Using Conditional
Expressions and Operation and Effect Thereof>>

[0066]Hereinafter, Conditional expressions 1 to 4 for further limiting the
basic structure of the imaging lens and the operation and effect thereof
will be described. The imaging lens according to this embodiment may
satisfy only one of Conditional expressions 1 to 4, or combinations of
two or more of Conditional expressions 1 to 4.

[0067]The meaning of each of the parameters represented by symbols in
Conditional expressions 1 to 4 will be described below: f indicates the
focal length of the entire imaging lens system, that is, the composite
focal length of the first lens L1 to the sixth lens L6; f1 indicates the
focal length of the first lens; f2 indicates the focal length of the
second lens; f36 indicates the focal length of the second lens group U2
(the composite focal length of the third lens to the sixth lens); R11
indicates the curvature radius of the image-side surface of the fifth
lens; R12 indicates the curvature radius of the object-side surface of
the sixth lens; and νd3 indicates the Abbe number of the third lens
with respect to the d-line.

[0068]The following Conditional expression 1 relates to the back focal
length or field curvature:

1.1<f36/f<1.7 [Conditional expression 1]

[0069]When the lens system is configured so as to satisfy Conditional
expression 1, it is possible to ensure a long back focal length without
increasing the total length of the lens system and prevent the occurrence
of field curvature.

[0070]However, when the lens system is configured such that the value of
f36/f is greater than the upper limit of Conditional expression 1, that
is, the value of f36/f is equal to or greater than 1.7, it is difficult
to ensure a long back focal length without increasing the total length of
the lens system.

[0071]On the other hand, when the lens system is configured such that the
value of f36/f is less than the lower limit of Conditional expression 1,
that is, the value of f36/f is equal to or less than 1.1, it is difficult
to correct field curvature.

[0072]It is more preferable that the value of f36/f satisfy Conditional
expression 1' given below:

[0074]When the lens system is configured so as to satisfy Conditional
expression 2, it is possible to effectively correct astigmatism, comatic
aberration, and lateral chromatic aberration.

[0075]However, when the lens system is configured such that the value of
f1/f2 is greater than the upper limit of Conditional expression 2, that
is, the value of f1/f2 is equal to or greater than -0.7, it is difficult
to correct astigmatism.

[0076]When the lens system is configured such that the value of f1/f2 is
less than the lower limit of Conditional expression 2, that is, the value
of f1/f2 is equal to or less than -1.4, it is difficult to correct
comatic aberration and lateral chromatic aberration.

[0077]It is more preferable that the value of f1/f2 satisfy Conditional
expression 2' given below:

-1.3<f1/f2<-0.8 [Conditional expression 2']

[0078]The following Conditional expression 3 relates to field curvature or
longitudinal chromatic aberration:

1.1<R11/R12<2.0 [Conditional expression 3]

[0079]When the lens system is configured so as to satisfy Conditional
expression 3, it is possible to effectively correct field curvature or
longitudinal chromatic aberration.

[0080]However, when the lens system is configured such that the value of
R11/R12 is greater than the upper limit of Conditional expression 3, that
is, the value of R11/R12 is equal to or greater than 2.0, it is difficult
to correct field curvature.

[0081]However, when the lens system is configured such that the value of
R11/R12 is less than the lower limit of Conditional expression 3, that
is, the value of R11/R12 is equal to or less than 1.1, it is difficult to
correct longitudinal chromatic aberration.

[0082]It is more preferable that the value of R11/R12 satisfy Conditional
expression 3' given below:

[0084]When the lens system is configured so as to satisfy Conditional
expression 4, it is possible to effectively correct longitudinal
chromatic aberration and lateral chromatic aberration.

[0085]However, when the lens system is configured such that the value of
νd3 is greater than the upper limit of Conditional expression 4, that
is, the Abbe number νd3 of a material forming the third lens L3 with
respect to the d-line is equal to or greater than 20, it is difficult to
correct longitudinal chromatic aberration and lateral chromatic
aberration.

[0086]As described above, according to this embodiment of the invention,
it is possible to obtain an imaging lens having a small size, a long back
focal length, and high environmental resistance.

Detailed Examples

[0087]Next, numerical data of the imaging lenses according to Examples 1
to 7 of the invention will be described with reference to FIGS. 2 to 15
and Tables 1 to 7. FIGS. 2 to 8 are cross-sectional views schematically
illustrating the structures of the imaging lenses according to Examples 1
to 7. In FIGS. 2 to 8, the same reference numerals as those in FIG. 1
denote the corresponding components.

[0088]The following Tables 1 to 7 shows basic data of the imaging lenses
according to Examples 1 to 7. In each of the tables, an upper part shows
lens data of the imaging lenses (which is represented by (a) in the
drawings) and a lower part shows the brief specifications of the imaging
lenses (which is represented by (b) in the drawings).

[0089]In the upper part in the lens data shown in Tables 1 to 7, the
surface number of an optical member, such as a lens, from the object side
is represented by an i-th (i=1 to 14) surface number, and the surface
number is sequentially increased toward the image side. In addition, the
lens data includes the surface number (i=5) of the aperture diaphragm St
and the surface number (i=14) of the imaging surface.

[0091]In addition, in the lens data, Ndj indicates the refractive index of
a j-th (j=1, 2, 3, . . . ) optical component from the object side with
respect to the d-line (wavelength: 587.6 nm). The number of the optical
component is sequentially increased toward the image side.

[0092]In addition, vdj indicates the Abbe number of the j-th optical
component with respect to the d-line.

[0093]The units of the curvature radius and the surface spacing are
millimeters (mm). When the lens surface is convex toward the object side,
the curvature radius thereof has a positive value. When the lens surface
is convex toward the image side, the curvature radius thereof has a
negative value.

[0094]The symbols of each item in the brief specifications shown in the
lower part of each of Tables 1 to 7 correspond to the following content.
Some of the following symbols have already been described above.

[0095]That is, the brief specifications of the following items are shown
in Tables 1 to 7: f indicates the focal length of the entire imaging lens
system (the composite focal length of the first lens to the sixth lens);
Fno indicates the F number; 2ω indicates a total angle of view; f36
indicates the focal length of the second lens group U2 including the
third lens, the fourth lens, the fifth lens, and the sixth lens; R11
indicates the curvature radius of the image-side surface of the fifth
lens, R12 indicates the curvature radius of the object-side surface of
the sixth lens; f1 indicates the focal length of the first lens, f2
indicates the focal length of the second lens; and νd3 indicates the
Abbe number of the third lens with respect to the d-line.

[0096]FIGS. 9 to 15 are diagrams illustrating all aberration of the
imaging lenses according to Examples 1 to 7. FIGS. 9 to 15 show the
aberrations of the imaging lenses according to Examples 1 to 7 with
respect to the e-line (wavelength: 546.1 nm), the g-line (wavelength:
435.8 nm), and the C-line (wavelength: 656.3 nm).

[0097]The distortion diagram shows the amount of deviation from an ideal
image height f×tan θ when the focal length of the entire lens
system is f and an angle of view is θ (a variable,
0≦θ≦ω).

[0098]As can be seen from the basic data of Examples 1 to 7 and the
diagrams illustrating various aberrations, according to the imaging lens
of the invention, it is possible to optimize the shape and material of
each of the six lenses. Therefore, it is possible to obtain an imaging
lens having a small size, a long back focal length, and high
environmental resistance.

[0099]Although the embodiment and the examples of the invention have been
described above, the invention is not limited to the embodiment and the
examples, but various modifications and changes of the invention can be
made. For example, the curvature radius, surface spacing, and refractive
index of each lens component are not limited to the values shown in the
drawings, but each lens component may have other values.

[0100]FIG. 16 is a diagram illustrating a vehicle provided with an
in-vehicle camera, which is an example of an imaging apparatus according
to the invention that includes the imaging lens according to the
invention and an imaging device converting the optical image formed by
the imaging lens into an electric signal.

[0101]As shown in FIG. 16, an in-vehicle camera 504 using the imaging lens
according to the invention is mounted to the rear surface of a room
mirror of a vehicle 501 and captures the front view of the vehicle 501.
When the vehicle 501 is travelling, the in-vehicle camera 504 captures
the front view to monitor the deviation of the vehicle 501 from its lane,
to monitor traffic signs, or to monitor whether there is an obstacle in
the course.